Many of various mobile electronic appliances such as cell phones, mobile information terminals PDA, note-type computers, DVD players, CD players, MD players, digital cameras, digital video cameras, etc. comprise batteries as power supplies, and DC/DC converters as power converters for converting power supply voltage to the predetermined operation voltage. The power supplies are conventionally disposed near input terminals and connected to loads of semiconductors (IC), etc., but recently problems such as response delay to current change due to the voltage reduction and current increase of semiconductors, voltage drop by wiring, and circuit impedance increase arise in a concentrated power supply system. Accordingly, a concentrated power supply system called POL (point of load), in which power supplies are disposed near loads, have got used. For instance, an isolated DC/DC converter is disposed near an input terminal, and a non-isolated DC/DC converter is disposed near a load.
FIG. 19 shows one example of non-isolated DC/DC converter circuits. This DC/DC converter is a step-down DC/DC converter comprising an input capacitor Cin, an output capacitor Cout, an inductor 1 (Lout), and an integrated semiconductor circuit IC comprising a control circuit, etc. DC input voltage Vin turns on a switching device (field effect transistor) in the integrated semiconductor circuit IC by a control signal from the control circuit. Output voltage Vout is reduced from input voltage Vin according to the formula of Vout=Ton/(Ton+Toff)×Vin, wherein Ton represents a time period in which the switching device is turned on, and Toff represents a time period in which the switching device is turned off. Even though there is variation in the input voltage Vin, stable output voltage Vout can be obtained by adjusting a ratio of Ton to Toff.
A conventional DC/DC converter circuit is a discrete circuit comprising a switching device, an integrated semiconductor circuit (active device) comprising a control circuit, and passive components such as inductors, capacitors, etc. mounted on a printed circuit board, etc., and the entire circuit has become a module for the miniaturization of electronic appliances. Higher switching speeds have become used to reduce output voltage variations, so that DC/DC converters operated at switching frequencies from 1 MHz to 5 MHz have become widely used recently, and further frequency increase has been pursued.
DC/DC converters are used in various environments, subjected to heat from the integrated semiconductor circuits IC and surrounding circuits. Accordingly, inductors in the DC/DC converters should have stable temperature characteristics. Also, because DC current superimposed with triangular-wave alternate current flows through the inductors, they should have excellent DC superimposition characteristics.
Because the voltage conversion efficiency of DC/DC converters are largely affected by loss generated in switching devices and inductors, magnetic ferrite used for inductors should have low loss near switching frequencies. Further, inductors should not suffer the variations of characteristics due to stress generated while molding resins are cured, thereby stably providing low loss.
Many of such inductors for DC/DC converters are conventionally of a so-called wire-wound type, in which copper wires are wound around ferrite cores. However, smaller wire-wound inductors make the production of cores more difficult with lower strength. Also, because magnetic fluxes leaking from the wire-wound inductors having open magnetic circuits affect surrounding circuit devices, circuit devices cannot be disposed near the inductors, making it difficult to reduce mounting areas.
As the electronic appliances such as mobile appliances are miniaturized with more functions, power supply circuits used therein are required to be smaller (smaller space and height). However, because wire-wound inductors occupy large areas and have large height, they are not adapted to the miniaturization of circuits by switching frequency increase. Accordingly, closed-magnetic-path multilayer inductors comprising conductor lines integral with magnetic ferrite to reduce leaked magnetic fluxes have got used.
Magnetic ferrite for inductors is required to have a high saturation magnetic flux density, with small change of saturation magnetic flux density and initial permeability in a use temperature range. As such magnetic ferrite, JP 2005-97085 A discloses Ni ferrite comprising 100% by weight of main components comprising 45.5-48.0% by mol of Fe2O3, 5-10% by mol of CuO, and 26-30% by mol of ZnO, the balance being substantially NiO, and 0.005-0.045% by weight (calculated as CoO) of cobalt oxide as a sub-component, both absolute values of a relative temperature coefficient of initial permeability (αμir) between −40° C. and +20° C. and αμir between 20° C. and 160° C. being 3 ppm/° C. or less, its quality coefficient (Q value) at 100 kHz being 170 or more, and its absolute value of stress resistance characteristics being 5% or less. Though this Ni ferrite has stable temperature characteristics of inductance, it is insufficient in the temperature characteristics of a saturation magnetic flux density Bs important to inductors for DC/DC converters.
FIG. 20 schematically shows the magnetization curves of magnetic ferrite having a small absolute value of αμir. In general, the saturation magnetic flux density Bs of magnetic ferrite decreases, as the temperature gets higher (temperature: T3>T2>T1) toward the Curie temperature Tc. Accordingly, though the temperature change of a saturation magnetic flux density is small in a low magnetic field H, it becomes drastically larger as the magnetic field H increases.
FIG. 21 shows the DC superimposition characteristics of an inductor comprising magnetic ferrite having such magnetization characteristics. Though small superimposed current Idc provides small inductance change with the temperature, large superimposed current Idc causes inductance to extremely decrease as the temperature gets higher. Accordingly, the control of DC/DC converters by feedback current is difficult, failing to achieve stable operation.